U.S. Pat. No. 4,724,894 by my colleague, Eric Sjodahl, discloses a method for controlling the pouring of molten metal in which a laser sensor is used to determine the height of metal being poured in to a mold. The instant invention is aimed at maximizing the accuracy (and thus productivity) of pouring control, particularly in casting aluminum, where unusual conditions exist both optically and mechanically which are not seen in iron casting applications.
In practicing the teachings of the Sjodahl invention, I have found that calibration of molten metal level sensors for mold level control is essential, as is their operation over a wide range of unusual reflection conditions, particularly when pouring aluminum billets. Accuracy demands are most stringent in this application.
Typically the mold is made of steel. Aluminum is poured in the top of the mold, the sides of which are fixed. The bottom of the mold is on a movable ram, which is lowered as the mold is filled, with the open sides cooling in the process in the region below the mold, as the ram recedes into a cooling tank. When the ram is at its bottommost position, the billet is of the desired length (height).
Water is sprayed on the sides of the mold which can create steam, even though most modern systems have steam evacuation systems sucking from below. In addition, at the beginning of the pour, the mold side walls have some grease on them, which creates smoke as the molten material vaporizes the grease.
An important operational parameter is the measured height of metal in the mold, which also relates to the speed of lowering the ram and the pour rate. For a given pour rate, the ram cannot be lowered too fast, or the molten metal level can sink below the edge of the mold creating an explosion. However, it is desired to cast as low as possible in the mold to get the best surface result, and this makes the task of keeping the accuracy through the entire cast (the task of pouring into a mold) extremely important. I have found electro-optical sensors, and particularly laser triangulation type electro-optical sensors very effective for monitoring molten metal height in this application. However, sensing is made difficult because of the steam and smoke conditions which can occur. For example a projected laser spot can hit both steam and the metal below it. With lots of steam, one can get a false high reading (i.e., closer to the sensor) from the steam. In other circumstances as well there is need to discriminate against readings from locations other than the metal level surface itself.
Measurement is also difficult at times due to turbulence in the molten metal and sometimes strange reflections that can occur due to the high reflectivity of the material, the motion thereof, and the formation of oxide or other layers on its surface.
Accurate height measurement today is also difficult because the current method to ensure calibration is made only prior to a given cast. If the sensor is mounted to a straight feeding launder (the last part of a continuous trough-like vessel used to transfer the molten aluminum from the furnace to the mold, which may be 6-7 m long), this feed launder quite often can move out of position due to thermal distortion or other factors during a cast, causing a loss of accuracy in height measurement due to movement of the sensor.
In the art there are many discussions of aluminum casting issues (see for example, U.S. Pat. No. 5,875,832 by Senser, et al.). And there is a good commercial description of the use of laser sensors for pouring control in aluminum produced by my employer, LMI Selcom, referenced herein in CD ROM form. However, I have been searching for a solution to the problems encountered, and have found no example of others effort in regard to the particular aspects mentioned above, whose solution is desirably here disclosed.
I have found that the problems of sensing in the presence of obscuration due to steam and smoke, and reflectivity changes due to formation of surface layer oxides and turbulence, as well as the problems of calibration, can be advantageously solved with a novel sensor and data processing approach here disclosed. Numerous readings in both time and lateral position across the metal are taken to prevent transient effects from being counted using algorithms disclosed herein. This can be aided by using multiple laser spots or a projected laser zone, such as a line, rather than a spot, to provide more range points from the surface of the molten metal, also at different locations across the mold.
Calibration of the system is insured by measuring both the molten metal height and the mold location during the pour. The differential height between the two then gives an accurate reading of metal height in the mold irregardless of feeding launder position or any other displacement of the sensor mounting position. Preferably, one sensor, or alternatively, two sensors (one for metal, one for mold) can be used for this purpose. The sensors employed are preferably, but not necessarily, electro-optically based. It is also noted that this approach, because it is independent of feed launder mounting position, is not affected by removal of the feed launder for maintenance, refurbishment, or other purposes. This simplifies set-up.
In many cases too it is desired to control height of metal in the feeding launder as well, also made possible with the invention.
It is therefore a goal of this invention to disclose an improved means for metal pouring control, particularly, but not necessarily, for aluminum casthouse application.
It is a further goal to provide improved means for pouring even in the presence of unusual reflections, caused by oxide formation, turbulence and other effects.
It is a still further goal to provide a method for assuring calibration of the sensor baseline location to provide accurate pouring control, in the presence of thermally induced mechanical distortion and other problems.
It is another goal of the invention to provide simplified means of calibrating the system after each cast, even when equipment is removed
It is still another goal to provide means for monitoring and controlling level in the feeding trough (launder) as well as molds being poured
These and other desirable features of the invention are illustrated in the following figures